This invention relates to the encoding of an analog signal into digital bits and, more particularly, to the type of encoding commonly known as pulse code modulaion (PCM).
A limited capacity of existing digital transmission systems coupled with an ever-increasing demand for transmitting informaion has given rise to several encoding techniques for decreasing the number of bits transmitted per unit time without a corresponding reduction in the subjective quality of the transmitted intelligence. Various encoding techniques have been devised to increase the information content of the digital bits used to encode each sample of the analog signal. This is true of conventional PCM and of a variation referred to as differential pulse code modulation (DPCM) wherein only changes in amplitude and sign of the difference between successive samples of an analog signal are encoded instead of the total amplitude of each sample as is done in PCM.
Early prior art techniques for the encoding of successive samples of an analog signal utilized a fixed code group of digital bits to characterize each sample. The presence or absence of a pulse or digital bit of information at each specific location or time slot in the digital code group provides a particular combination that conveys the intelligence information which is usually indicative of one of several discrete steps known as the quantized amplitude of the sample. The number of time slots in a digital code group is a factor in determining the number of combinations or different quantized amplitude levels which the digital group can represent. Generally, a common procedure in conventional differential pulse code modulators is to assign one bit in each digital code group to represent the polarity or sign of each sample.
An improved differential encoding technique has been devised wherein actual polarity changes between samples are signaled by a prescribed digital group called a flag word to take advantage of the reduced number of polarity changes which only occur between approximately every fourth sample. Then only the absolute amplitude or magnitude of each sample is necessary to be encoded which reduces the overall number of digital bits required to encode the analog signal. The common disadvantage of all prior art techniques is that extra digital bits are required in the encoded signal to indicate sign information.
A primary object of this invention is to eliminate the conventional necessity of using extra digital bits to encode polarity or sign information of samples of an analog signal.